The invention relates to measuring of breath gases of a patient. The patient's breath gases are measured for example in connection with a ventilation procedure carried out with the patient. The term ventilation covers here both ventilation procedures and also anesthesia procedures carried out with the patient.
A widely known way to measure breathing gases from the patient is to use sidestream technique. In this technique a ventilator is connected with ventilator tubes to the patient. Y-piece is connected to the airway adapter, which includes sampling line connector. Then there is a filter and intubation tube. Breath gas measuring unit draws a small amount of breathing air from the airway adapter using a pump. Air flows through the sampling line to the water trap, which removes water and mucus from the air. The air flows to the gas sensor, which measures one or more breath gases. Then there is a sample flow measurement unit and a pump. The CPU determines the gas concentrations based on the sensor signal(s). It also determines the sample flow and controls the pump according to the measurement result.
In this known technique normal way to implement sample flow control is to use a constant sample flow rate, which means that regardless of the situation the sample flow rate is tried to keep constant. The amount of sample flow rate is determined to be high enough for the maximum specified breathing rate that the device is enabled to measure.
The main drawbacks in the constant sample flow rate can be described as follows.
Because the sample flow rate may be higher than needed more water and mucus are drawn from the breathing circuit than it is necessary to achieve good measurement accuracy. Because of that the lifetime of the sampling line and water trap gets shorter. Also there are more occlusion situations than in the case of smaller sample flow rate.
The high sample flow shortens the lifetime of the pump. Also the noise generated by the pump is proportional to the load of the pump.
While using unnecessary high flow sample rate, in case of anesthesia the device draws unnecessary amount of expensive anesthetic gases from the ventilator circuit.